RIAA Preamps Part 2 - Tube CAD Journal

+210V
5.5k
8k
11k
+100V
.0099µF
+130V
117k
+130V
.47µF
300 32.5k
6DJ8 +2V 6DJ8 +2V
300
.03µF
6DJ8 +24V
.1µF 300
6DJ8 +2V
1M
47k
200
200
8.7k
24k
1M
200
Cascading these two functions results in the
same curve as the traditional two function
combination creates. So, why bother with this
alternate approach Beyond the mental
stretching, which is always for the good, this
alternative approach engenders new phono
preamp topologies, with some interesting
benefits and one negative. The first benefit is
that the 50Hz low-pass filter can filter more
power supply noise away from the output signal
than the 2122Hz can.
.1µF
1M
+10V
200
47k
Ra
1200
10k
91k
R1
C1
+12V
C2
Rk
.266µF
+210V
.0318µF
A single-triode phono preamp
Second, the 500Hz to 2122Hz shelving
network can be implemented within one
grounded–cathode amplifier, i.e. a single triode
RIAA equalized preamp. Usually a cathode
resistor is bypassed with a large valued
capacitor, so as to ensure adequate lowfrequency
response. But if the bypass capacitor’s
value is reduced greatly, it introduces a shelving
function, wherein the high-frequency gain is
greater than the low-frequency gain, with the
ratio being equal to the bypassed gain over the
unbypassed gain or
1 .
Ratio = 1 + (mu + 1)R k
R a + r p
The transition frequencies are based on the
capacitor’s value and the time constants it forms
with the triode’s r p and its plate and cathode
resistor values:
R k C k = 75µS
[(R a + r p )/(mu + 1) || R k ]C k = 318µS
Of course, one triode is not going to provide
enough gain in most cases, so additional gain
stages will be needed.
The overarching liability of this different
path to RIAA equalization is the –12dB insertion
loss beyond the expected losses, which brings it
1kHz insertion loss down to –32dB. With this
equalization network, the near DC and subsonic
frequencies are reduced by –12dB, whereas the
traditional path retained all of the gain at the
bottom of range. Now, -12dB is just too much to
pay in most two-gain-stage phono preamps, but
it is almost nothing in a three-gain-stage preamp,
where the problem is usually having too much
gain. Still the advantage of using a portion of the
equalization network in double duty either, as a
pseudo coupling capacitor or a partial cathodebypass
capacitor, is tempting.
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